The present invention relates to a process and a circuit for exciting an ultrasonic generator, and to the use of the circuit with the ultrasonic generator for atomizing a liquid.
The possibility of atomizing liquids with the aid of piezoelectric ultrasonic generators is known. For example, the article by W.D. Drews "Flussigkeitszerstaubung durch Ultraschall" in Elektronik, 1979, No. 10, pp. 83 to 90, briefly describes the principle of this process, in which an ultrasonic generator or vibrator is provided with an atomizer disk or plate and a circuit for exciting the ultrasonic generator.
However, several problems are encountered in technically bringing about the atomization of a liquid with the aid of an ultrasonic generator.
As atomization is only possible close to the resonance of the unit comprising the ultrasonic generator and its atomizer plate, it is necessary to very accurately maintain the required exciting frequency. The locking of the oscillator of the exciting circuit to an apparent resonance, not corresponding to an effective atomization, must be reliably prevented.
The exciting circuit must be in a position to follow the exciting frequency corresponding to the changes of different parameters. Such parameters are e.g. the manufacturing tolerances of the mechanical components of the ultrasonic generator and in particular its atomizer disk, the variations to the mechanical and electrical parameters of the piezoceramics used for its production, the operating temperature of the ultrasonic generator (very important when used in burners), the aging of the ultrasonic generator, the deposits formed thereon (such as e.g. soot and resins when used in burners), the characteristics of the medium to be atomized and the manufacturing, adjustment and other tolerances in the exciting circuit.
It must be possible to reliably detect the stopping of atomization. If the stoppage is caused by droplets sticking to the atomizer disk, a removal of the droplets from the disk must be ensured.
A practical requirement concerning the industrial usability is the free interchangeability of the exciting circuit and the ultrasonic generator or optionally its atomizer disk without any adjustments and without high tolerance requirements on the spares of the components of the circuit (which is particularly important when individual components are replaced for repair purposes).
The active power of the ultrasonic generator or its atomizer disk must be regulatable in a very wide dynamic range and the control of the active power and the frequency must not reciprocally influence one another. In addition, changes to the aforementioned parameters and the operating voltage must have little or no influence on the operation of the control loops.
Various processes and circuits have already been proposed for solving these problems.
DE-3222425 proposes exciting the ultrasonic generator by means of a matching network, which inter alia suppresses the oscillation of the ultrasonic generator to harmonics of its resonant frequency. The direct current component of the resonator current is used for regulating the exciting current and the alternating current component of the resonator current is used for regulating the exciting frequency, a low-pass filter only allowing the frequency component to pass to the desired resonant frequency of the ultrasonic generator. In the case of resonance failure, the exciting frequency is wobbled or swept, in order to pass through the resonance point and achieve relocking. However, it is a disadvantage of this solution that the circuit must be matched to the ultrasonic generator and in particular to its desired resonant frequency, so that the operation of the ultrasonic generator cannot follow the changes of certain of the aforementioned parameters and the necessary easy interchangeability of the components is not ensured. A reliable operation during oscillation, particularly under load and under varying operating conditions, is not ensured, because the impedance and therefore the phase relationships between the current and voltage of the ultrasonic generator vary considerably in the case of load changes and therefore a following of the optimum oscillation frequency, derived from phase relationships between the current and voltage in the ultrasonic generator, is not possible. It is not possible to achieve a true compensation of the capacitance of the ultrasonic generator by means of an inductance coil, due to the capacitance which varies during operation and particularly during load changes.
A somewhat different construction is proposed in U.S. 4,275,363, but the same, aforementioned disadvantages occur.
DE-3534853 proposes operating the ultrasonic generator with timed excitation power (bursts) and carrying out a current measurement at specific times for automatic frequency matching. The necessary intermediate storage of the current measurement values and the precise synchronization of the control sequences are particularly disadvantageous and costly.
Swiss patent application 3155/87-0 of 8-17-1987 inter alia proposes, with a constant exciting voltage at the ultrasonic generator, to regulate the power by means of a variation of the operating frequency in the frequency range between the series resonance and the parallel resonance of the ultrasonic generator. The amplification of the control loop intended for this purpose is such that slight control oscillations occur if the ultrasonic generator oscillates in undamped manner. However, if the control oscillations are missing, a wobbulator or sweep generator is locked on, in order to shake off any droplets attached to the ultrasonic generator and to again seek the operating frequency. It is a disadvantage that in the case of ultrasonic generators which do not reach the maximum settable power, the exciting circuit in the case of a high power demand can no longer be locked on the desired operating frequency. A further disadvantage is that the current/frequency characteristic has a limited steepness close to the parallel resonance, i.e. at low power levels, which leads to a stoppage of the control oscillation and therefore to inappropriate locking on of the sweep generator. The dynamic range of the power control is greatly restricted by these two phenomena.